1. Field of the Invention
The present invention relates to dispersions of mineral fillers, particularly calcium carbonates, and dispersing agents. Cement matrices or hydraulic binders, and more particularly hydraulic concretes, prepared with the dispersions of mineral fillers and dispersing agents of the present invention have an appreciably improved mechanical property known as “strength at young ages.”
The present invention also relates to mortars, concretes and other compositions based on cement and/or semi-hydrate calcium sulphate, i.e., hydraulic compounds or binders.
2. Discussion of the Background
It is known that cement matrices such as concretes, mortars, grouts, etc., more particularly hydraulic concretes, are employed in two main types of applications, ready-for-use concretes and prefabricated products.
Concrete is composed essentially of a cement and an aggregate as well as water and admixtures or additives. Aggregates are classed in several categories known to experts and defined by the French standard XP P 18-540. In this standard, the families of aggregates comprise:                0/D fillers where D<2 mm, with at least 70% of the filler passing through a 0.063 mm screen,        0/D fine sands where D<1 mm, with less than 70% of the fine sand passing through a 0.063 mm screen,        0/D sands where 1≦D 6.3 mm,        gravels where D>6.3 mm,        d/D fine gravels where d>1 mm and D 125 mm,        d/D ballasts where d>25 mm and D 50 mm,with d and D as defined in the standard.        
Examples of fillers include, for example, fumed silica or siliceous additives, or calcareous additives such as calcium carbonate. It is known that calcium carbonate can be used either in the form of a dispersion in water or in an aqueous medium and not including any dispersant.
WO 99/47468 describes the use of calcium carbonate in the form of an aqueous dispersion for preparing concretes. This document describes a more or less acceptable compromise between ease of handling and resistance to premature aging. However, WO 99/47468 only describes incorporating, in the cement matrix or hydraulic binder, an additive used as a liquefier for improving fluidity, which is similar to the known functions of additives in the prior art.
EP 0 271 435, EP 0 725 043 and U.S. Pat. No. 5,614,017 describe the use of plasticizers in cements in order to improve the compressive strength, or reduce shrinkage, or increase the workability time of cement, measured with the Abrams cone, also referred to as “slump.” In EP 0 271 435, the additive acts as a water reducer, in order that the water/cement (W/C) ratio be as low as possible so that the mechanical strength of the cement improves. In EP 0 725 043, the plasticizer is used at a very low concentration and the technical problem posed is completely different from that of the present invention, as with U.S. Pat. No. 5,614,017. In these three documents, the aggregate is of the 0/D filler type as defined above (D<2 mm with at least 70% of the aggregate passing through a 0.063 mm screen), the calcium carbonate is used as a dry powder rather than in dispersion, and the admixture is added at the time of preparation of the cement matrix or hydraulic binder, that is to say subsequently to, rather than before the preparation of the cement matrix or hydraulic binder, as in the present invention.
It is also known that admixtures may be used for deflocculating the cement paste and/or reducing the quantity of water in the cement. In this regard, if the proportion of water increases in a cement matrix or hydraulic binder, the mechanical strength is reduced. Experts therefore always try to reduce the water/cement W/C ratio in their formulations. However, experts also know that if the proportion of water increases in a cement matrix or hydraulic binder, the workability of the cement matrix or hydraulic binder (that is to say its ability to be handled, pumped, etc) increases. Experts are therefore very often forced to seek a compromise between the mechanical properties of the cement, and its workability.
Among the mechanical properties of cement matrices or hydraulic binders, one of the most important properties for experts concerns “strength at young ages.” This property is defined as the change in the compressive strength curve, as a function of the age of preparation of the cement matrix or hydraulic binder, in the time period of from 0 hours to 7 days following the preparation of the cement matrix or hydraulic binder, in particular, measured at a standard time of 2 days and 7 days.
In the prior art, as discussed above, this property cannot be improved, but must be a compromise with other properties. Like any compromise, it is not satisfactory. There is therefore an important and recognized need for significantly increasing the property of strength at young ages, without negatively affecting other desired properties, such as, for example, workability.